This invention relates to a reactive flux useful in joining by brazing surfaces of aluminum and aluminum alloy (hereinafter referred to collectively as "aluminum articles") to one another and to other metal articles, such as copper and stainless steel, for example, and to methods of brazing employing such flux.
Generally, the technique of brazing has been employed in assembling metal articles, consisting wholly or in part of aluminum or aluminum alloys, the parts of which are joined by means of a brazing alloy, which has a lower melting point.
In the construction of automotive parts, for example, the technique of brazing has been employed for the manufacture of structural members such as intake manifolds and cylinder heads as well as heat exchangers for condensers, evaporators, and engine oil coolers. The technique has also been adopted for the manufacture of rectifier heat sinks for use in electronic devices. In various other industrial fields, it has also found widespread acceptance.
When, for example, two aluminum articles are to be joined by means of a brazing alloy, a flux is used to remove any oxide film on the surfaces to be joined. At the same time, the flux improves the wet spreading or flowing property of the molten brazing alloy over the surfaces to be joined. Chloride fluxes have been widely used for that purpose.
Since a chloride flux is soluble in water and hygroscopic, it has a disadvantage that when the flux itself, or the reaction residue resulting from brazing, is permitted to adhere to and remain on the articles being joined, it causes corrosion of the aluminum articles. For this reason, where brazing has been effected with a chloride flux, the brazing operation has had to be followed by a step to remove the residual flux. If some part of the brazed joint being cleaned happens to be inaccessible, there is a possibility that some flux will remain there and eventually lead to corrosion.
It is already known that the corrosion problem arising from the use of chloride fluxes in a brazing operation can be overcome by the use of a flux based on the AlF.sub.3 -KF binary system which leaves no water-soluble residues.
It is already known from U.S. Pat. No. 3,951,328 to employ fluxes of that type which are free from water-soluble components; particularly fluxes which are free from unreacted KF. Such fluxes can be employed in the form of an aqueous slurry, which is dried off before the commencement of the brazing operation. Since the brazing operation is very advantageously carried out in a very dry atmosphere, particularly a very dry nitrogen atmosphere, it is extremely desirable that the applied flux should be free of hygroscopic components, such as unreacted KF.
The flux specifically described in U.S. Pat. No. 3,951,328 consists of a mixture of potassium fluoaluminates, which may be formed separately and mixed or be formed in intimate mixture by a separately performed chemical reaction.
It was stated in U.S. Pat. No. 3,951,328 that the flux could include up to about 5 mole % total of LiF, CaF.sub.2, NaF, but such addition was stated to result in an increase in the liquidus temperature ("melting point") of the flux and it was therefore concluded that such addition of alkali metal fluoride or alkaline earth metal fluoride was not advantageous.
The lowest liquidus temperature of the straight potassium fluoaluminate flux of U.S. Pat. No. 3,951,328 is about 560.degree. C. (substantially higher than that of the chloride-type fluxes already mentioned). Such a liquidus temperature is considered too high for use in brazing operations where one of the components has a relatively low solidus temperature, for example where it is formed from an Al casting, having a high Si content. Generally, it is considered desirable in a brazing operation, in order to achieve a stable product quality and a reliable brazing operation, that, on the one hand, the melting point (liquidus) of the brazing alloy should be 10.degree. to 40.degree. C. lower than the solidus temperature of the articles to be joined, while on the other hand, the melting point (liquidus) of the flux should be equal to or up to 20.degree. C. lower than the solidus temperature of the brazing metal. From this point of view, any reduction in the melting point of the flux itself can be expected to expand the range of aluminum alloys which can be joined by brazing and, as a consequence, widen the range of useful products that can be assembled in this manner.
Further in the case of the known potassium fluoaluminate flux, if this flux is used for brazing aluminum articles containing magnesium, the wet spreading or flowing property exhibited by the brazing metal on such aluminum articles suffers and the brazing operation itself may be impaired so much as to render achievement of the desired joint strength impracticable. From a practical point of view, therefore, this flux has been considered applicable to the brazing of aluminum alloys having a magnesium (Mg) content of no more than 0.4% (by weight).
In U.S. Pat. No. 3,951,328, the relative content of the potassium fluoaluminate in terms of AlF.sub.3 and KF were stated to be (by weight) AlF.sub.3 65 -45% and KF 35-55% with a preferred composition of about AlF.sub.3 54.2% and KF 45.8%.